Atmos. Chem. Phys. Discuss., 7, 14209-14232, 2007
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
When does new particle formation not occur in the upper troposphere?
D. R. Benson1, L.-H. Young1, S.-H. Lee1, T. L. Campos2, D. C. Rogers2, and J. Jensen2
1Kent State University, Department of Chemistry, Kent, OH, USA
2National Center for Atmospheric Research, Earth Observing Laboratory, Broomfield, CO, USA

Abstract. Recent aircraft studies showed that new particle formation is very active in the free troposphere and lower stratosphere. And, these observations lead to a new question: when does new particle formation not occur? Here, we provide case studies to show how convection and surface area affect new particle formation in the upper troposphere, using the measured aerosol size distributions during the NSF/NCAR GV Progressive Science Missions in December 2005. There were ten research flights, including three days of nighttime experiments, at latitudes from 18 to 52° N and altitudes up to 14 km. About 78% of the total samples showed the new particle formation feature with number concentrations of particles with diameters from 4 to 9 nm, 670±1270 cm−3, and the total particle number concentrations with diameters from 4 to 2000 nm, 920±1470 cm−3. Our case studies show that new particle formation was closely associated with convection and low surface areas of preexisting aerosol particles (<4 μm² cm−3). On the other hand, for the cases where no new particle formation events were observed, air masses usually did not experience a vertical motion and air often originated from either the upper troposphere or lower stratosphere where precursor concentrations are relatively low; in addition, it was also a general trend that non-event cases also had higher surface areas (~16 μm² cm−3). These observations are consistent with other observations during the Progressive Science Missions (Young et al., 2007). Because of the lower temperatures in this region (T<250 K), nucleation is thermodynamically favorable; but because of low aerosol precursor concentrations, nucleation is sensitive to aerosol precursor concentration and surface area. Under such conditions, convection (which brings higher concentrations of aerosol precursors and water vapor to higher altitudes) and low surface area play critical roles on whether new particle formation takes place or not. Latitude dependence of new particles also shows higher particle concentrations in the midlatitude tropopause region than in the subtropics, consistent with Hermann et al. (2003).

Citation: Benson, D. R., Young, L.-H., Lee, S.-H., Campos, T. L., Rogers, D. C., and Jensen, J.: When does new particle formation not occur in the upper troposphere?, Atmos. Chem. Phys. Discuss., 7, 14209-14232, doi:10.5194/acpd-7-14209-2007, 2007.
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